std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,std::llround,std::llroundf (3) - Linux Manuals

std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,std::llround,std::llroundf: std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,std::llround,std::llroundf

NAME

std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,std::llround,std::llroundf - std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,std::llround,std::llroundf

Synopsis

Defined in header <cmath>
float round ( float arg ); (1) (since C++11)
float roundf( float arg );
double round ( double arg ); (2) (since C++11)
long double round ( long double arg ); (3) (since C++11)
long double roundl( long double arg );
double round ( IntegralType arg ); (4) (since C++11)
long lround ( float arg ); (5) (since C++11)
long lroundf( float arg );
long lround ( double arg ); (6) (since C++11)
long lround ( long double arg ); (7) (since C++11)
long lroundl( long double arg );
long lround ( IntegralType arg ); (8) (since C++11)
long long llround ( float arg ); (9) (since C++11)
long long llroundf( float arg );
long long llround ( double arg ); (10) (since C++11)
long long llround ( long double arg ); (11) (since C++11)
long long llroundl( long double arg );
long long llround ( IntegralType arg ); (12) (since C++11)

1-3) Computes the nearest integer value to arg (in floating-point format), rounding halfway cases away from zero, regardless of the current rounding mode.
5-7, 9-11) Computes the nearest integer value to arg (in integer format), rounding halfway cases away from zero, regardless of the current rounding mode.
4,8,12) A set of overloads or a function template accepting an argument of any integral_type. Equivalent to 2), 6), or 10), respectively (the argument is cast to double).

Parameters

arg - floating point value

Return value

If no errors occur, the nearest integer value to arg, rounding halfway cases away from zero, is returned.

Return value

 math-round away zero.svg
Argument
If a domain error occurs, an implementation-defined value is returned

Error handling

Errors are reported as specified in math_errhandling.
If the result of std::lround or std::llround is outside the range representable by the return type, a domain error or a range error may occur.
If the implementation supports IEEE floating-point arithmetic (IEC 60559),

      For the std::round function:

* The current rounding_mode has no effect.
* If arg is ±∞, it is returned, unmodified
* If arg is ±0, it is returned, unmodified
* If arg is NaN, NaN is returned

      For std::lround and std::llround functions:

* FE_INEXACT is never raised
* The current rounding_mode has no effect.
* If arg is ±∞, FE_INVALID is raised and an implementation-defined value is returned
* If the result of the rounding is outside the range of the return type, FE_INVALID is raised and an implementation-defined value is returned
* If arg is NaN, FE_INVALID is raised and an implementation-defined value is returned

Notes

FE_INEXACT may be (but isn't required to be) raised by std::round when rounding a non-integer finite value.
The largest representable floating-point values are exact integers in all standard floating-point formats, so std::round never overflows on its own; however the result may overflow any integer type (including std::intmax_t), when stored in an integer variable.
POSIX_specifies that all cases where std::lround or std::llround raise FE_INEXACT are domain errors.
The double version of std::round behaves as if implemented as follows:

  #include <cmath>
  #include <cfenv>
  #pragma STDC FENV_ACCESS ON
  double round(double x)
  {
      std::fenv_t save_env;
      std::feholdexcept(&save_env);
      double result = std::rint(x);
      if (std::fetestexcept(FE_INEXACT)) {
          auto const save_round = std::fegetround();
          std::fesetround(FE_TOWARDZERO);
          result = std::rint(std::copysign(0.5 + std::fabs(x), x));
          std::fesetround(save_round);
      }
      std::feupdateenv(&save_env);
      return result;
  }

Example

// Run this code

  #include <iostream>
  #include <cmath>
  #include <cfenv>
  #include <climits>

  #pragma STDC FENV_ACCESS ON

  int main()
  {
      // round
      std::cout << "round(+2.3) = " << std::round(2.3)
                << " round(+2.5) = " << std::round(2.5)
                << " round(+2.7) = " << std::round(2.7) << '\n'
                << "round(-2.3) = " << std::round(-2.3)
                << " round(-2.5) = " << std::round(-2.5)
                << " round(-2.7) = " << std::round(-2.7) << '\n';

      std::cout << "round(-0.0) = " << std::round(-0.0) << '\n'
                << "round(-Inf) = " << std::round(-INFINITY) << '\n';

      // lround
      std::cout << "lround(+2.3) = " << std::lround(2.3)
                << " lround(+2.5) = " << std::lround(2.5)
                << " lround(+2.7) = " << std::lround(2.7) << '\n'
                << "lround(-2.3) = " << std::lround(-2.3)
                << " lround(-2.5) = " << std::lround(-2.5)
                << " lround(-2.7) = " << std::lround(-2.7) << '\n';

      std::cout << "lround(-0.0) = " << std::lround(-0.0) << '\n'
                << "lround(-Inf) = " << std::lround(-INFINITY) << '\n';

      // error handling
      std::feclearexcept(FE_ALL_EXCEPT);
      std::cout << "std::lround(LONG_MAX+1.5) = "
                << std::lround(LONG_MAX+1.5) << '\n';
      if (std::fetestexcept(FE_INVALID))
                std::cout << " FE_INVALID was raised\n";
  }

Possible output:

  round(+2.3) = 2 round(+2.5) = 3 round(+2.7) = 3
  round(-2.3) = -2 round(-2.5) = -3 round(-2.7) = -3
  round(-0.0) = -0
  round(-Inf) = -inf
  lround(+2.3) = 2 lround(+2.5) = 3 lround(+2.7) = 3
  lround(-2.3) = -2 lround(-2.5) = -3 lround(-2.7) = -3
  lround(-0.0) = 0
  lround(-Inf) = -9223372036854775808
  std::lround(LONG_MAX+1.5) = -9223372036854775808
      FE_INVALID was raised